package_body ppp
aaa clob;
...
procedure change_content()
begin
...
aaa := replace(aaa, 'b', ';;');
...
end;
procedure call_f()
begin
aaa := empty_clob();
aaa := 'aaaaaaaaaaaaaaaaaaaabaaaaaaaaaaaaaaaaaaaaabaaaaaaaaaaaaaaaaaaa’
for i in 1..10000 loop
change_content();
end loop;
end;
从逻辑上看，这段代码可以使用下面的代码进行模拟，
declare
aaa clob;
begin
aaa := empty_clob();
aaa := 'aaaaaaaaaaaaaaaaaaaabaaaaaaaaaaaaaaaaaaaaabaaaaaaaaaaaaaaaaaaaa’
for i in 1..10000 loop
aaa := replace(aaa, 'b', ';;');
end loop;
end;
许多人都知道，在PLSQL中的LOB类型变量是占用临时表空间的。但是，从以上代码看，CLOB变量aaa的初始值并没有占用太大空间。我们在测试环境上运行该语句，用以下语句观察其临时表空间占用情况。
SELECT b.tablespace,
ROUND(((b.blocks*p.value)/1024/1024),2)||'M' "SIZE",
b.CONTENTS,
s.sql_text,
a.sid||','||a.serial# SID_SERIAL,
a.username,
a.program,
sysdate as log_date
FROM sys.v_$session a,
sys.v_$sort_usage b,
sys.v_$parameter p,
v$sqlarea s
WHERE p.name = 'db_block_size'
AND a.saddr = b.session_addr
AND a.sql_address = s.address and a.sql_hash_value = s.hash_value
ORDER BY b.tablespace, b.blocks;
TABLESPACE SIZE CONTENTS SQL_TEXT SID_SERIAL USERNAME PROGRAM LOG_DATE
---------------------------------------------------------------------------------
TEMP 37M TEMPORARY declare aaa clob; begin aaa := empty_clob(); aaa := 'aaaaaaaaaaaaaaaaaaaabaaaaaaaaaaaaaaaaaaaaabaaaaaaaaaaaaaaa';
for i in 1..1000000000 loop aaa := replace(aaa, 'a', ';;'); end loop; end; 137,28675 DEMO plsqldev.exe 8/15/2007 11:42:11 AM
发现临时表空间在不停增长。问题就在于aaa := replace(aaa, 'b', ';;').Replace()函数会先将传入的CLOB参数变量在temp空间上保存一份，然后再转换为varchar2类型进行操作。这样，每次进行了replace操作后，就多占据了一份temp空间。
大家可以通过包dbms_lob中提供的copy、instr和write函数来自己编写一个clob的字符替换函数，来避免temp空间的泄露。以下存储过程实现了CLOB中的字符串替换。
create or replace procedure lob_replace( p_lob in out clob,
p_what in varchar2,
p_with in varchar2 )
as
n number;
len number;
begin
n := dbms_lob.instr( p_lob, p_what );
while ( nvl(n,0) > 0 ) loop
len := dbms_lob.getlength(p_lob);
if (n+length(p_with)-1 > len)
then
dbms_lob.writeappend( p_lob, n+length(p_with)-1 - len, p_with );
end if;
if (len-n-length(p_what)+1 > 0)
then
dbms_lob.copy( p_lob,
p_lob,
len-n-length(p_what)+1,
n+length(p_with),
n+length(p_what) );
end if;
dbms_lob.write( p_lob, length(p_with), n, p_with );
if ( length(p_what) > length(p_with) )
then
dbms_lob.trim( p_lob,
dbms_lob.getlength(p_lob)-(length(p_what)-length(p_with)) );
end if;
n := dbms_lob.instr( p_lob, p_what );
end loop;
end;
结论：
使用上面的过程代替replace函数，再次运行程序，用以上语句进行观察，不再存在临时空间泄露的情况。
